Engine fan control output

[Hamish]

My CM870 only delivers 8 or 9 volts to the engine fan solenoid. I think this is the cause of very lethargic fan turn off (and on), which will wear out the hub quickly as it slips during the transition.

I have checked the relays (2007 International 9900) and they are fine. Is this a failed output driver in the ECM?

I could just wire it old school using a coolant temp switch and the AC high pressure switch, but do I have a bigger problem with the ECM?
Are there any other issues that will develop because the ECM may be failing? Does this warrant an ECM swap? Can this even be done without programming if the part number, cpl, and program version are identical? (I could swap one from another truck I own or just buy a used one if I find one)

ps. Searching the forum didn't seem to be working - i was pretty sure I saw relevant posts about this in the past...

[Rawze]

is it an all electric clutch?... or air operated and solenoid?. -- could even be a variable speed clutch. you did not specify.

[Hamish]

Air operated fan with electric solenoid controlling air supply. Air supplied to hub manually turns it off normally, and air removed turns it on normally (ie. Without lag or slipping)

[Hamish]

Assumed it was the solenoid as I've had them get sticky and fail, but installed a new one and no change which led to checking the voltage being delivered to the solenoid using a jumper I made with bared wires so I could connect a Fluke meter

[Rawze]

With what you described and ...

(05-03-2025 )Hamish Wrote:  My CM870 only delivers 8 or 9 volts to the engine fan solenoid. ...

just some speculation ...
It sounds to me like either the ECM itself has low voltage going to it, or fan clutch options maybe have been set for an electric fan clutch (can be seen in Insite settings), or its a bad output from the ecm (damage to the circuit internally).

[Hamish]

Good point to check the voltage being delivered to the ECM - I guess I had assumed it wouldn't even fire up if voltage was that low. Will certainly check that next before doing anything else

[DVT873]

(05-04-2025 )Hamish Wrote:  Good point to check the voltage being delivered to the ECM - I guess I had assumed it wouldn't even fire up if voltage was that low. Will certainly check that next before doing anything else

If your built after Jan of 2007 you may have another module in the mix. January 07 and up is S08339 and found at this site http://bodybuilder.navistar.com/General/BBcircuitIX.htm
Or just search for International Body Builder wiring. Prior to 07 they were 8600's as far as manuals went.
I have not tangled with a fan speed module but a quick look at the wiring shows some grounds. It looks like the body computer gets involved also. If it is newer tha Jan 2007. Post the VIN and I can see the build info.
Prior to 2007 it is just on/off from looking at the manual.

[Fleetman13]

I had an ECM that had a bad fan circuit driver. So it’s very possible this could be the case. I’d check all your power and grounds going into the ECM and make sure they are good. Make sure the circuits can carry a load (have integrity) don’t use a LED test light or a power probe they can send you down a rabbit hole some times. If all that checks out back probe you’re OEM harness at the ECM and make sure it’s sending out the correct voltage. If it’s not I’d load test your wires from your ECM to your fan if they test good then it’s a good possibility it’s the ECM. I wanna say the one I had issues with put out like 8.7 volts or something like that never could fire off the fan but it never had any other issues other than that. Also if you don’t have a circuit load tested then just wire up a head light bulb in between your circuits and if it will light that bulb your circuit has plenty of integrity.